1. Field of Invention
This invention relates to x-ray tomography. More generally it relates to an apparatus and method for acquiring three dimensional images showing the composition and shape of the interior of an object. More particularly, it provides three dimensional images derived from Compton scattered x-ray signals measured by several detectors that result when the object is irradiated with a substantially point source of ultrashort x-ray pulses. The invention also relates to the apparatus for producing and detecting such point source ultrashort x-ray pulses.
2. Prior Art
Currently most x-ray scanners use a tomographic technique for creating images. This involves measuring the amount of energy lost along many different lines of sight that all lie in a single slab. The slab is broken up into small volumes, or pixels, and based on the line of sight information, the amount of energy lost in each pixel is calculated. The cross section and density information for that pixel is then recovered. In practice, this results in a two dimensional image that has a certain thickness. Three dimensional information is recovered by moving the object that is being imaged.
Another three dimensional technique has been proposed based on the concept of photon migration. See, for example, U.S. Pat. No. 4,857,748 issued Aug. 15, 1989. This entails radiating a sample with ultrashort optical pulses and measuring the time evolution of the transmitted and reflected signals along a line of sight. Both of these signals include not only directly transmitted or reflected photons, but also photons which may be scattered several times inside the material and then reemerge. These photons are said to be diffusing or migrating through the object and might contain information regarding the structure of the object. This approach suffers from several problems. First and foremost, most materials are too optically "thick" to provide any information. Additionally, optical photons can have very complex paths before they reemerge and the mathematical unfolding of these signals to determine the structures that have scattered or reflected them is virtually an intractable problem.
Methods are also known for recognizing in photographic data curves having pre-determined configurations. For example U.S. Pat. No. 3,069,654 to Hough, issued Dec. 18, 1962, describes apparatus for determining the presence of straight lines in a photograph. The original use of this technique was to automate the determination of the presence of linear particle tracks in bubble chamber photographs.
The Hough invention recognizes the presence of markers in a photograph lying along a line in the photograph by first associating each unique marker in the photograph with a corresponding unique line in a second space. The association is done so that the lines in the second space form bundles passing through points (called knots) in the second space only if the associated markers lie on a straight line. Thus by looking for knots in the second space one locates all the lines on which the markers lie.
In particular if (x,y) is the location of the marker, then according to the Hough patent the associated line in the second space is defined by the equation y'=(x'-x)/y. (See column 1, line 70 to column 2, line 7). It's then a simple matter of algebra to show that a collection of markers satisfying the linear relationship y=mx+b has associated lines that all intersect at the so-called "knot" (x',y')=(-b/m, -l/m). Once the coordinates of the knot (x',y') are determined by inspection of the lines in the second space, the line in the photograph along which the markers lie is determined by the equation y=-x/y'+x'/y'.
The above association is known as a Hough transformation and has been generalized for curves other than straight lines. See for example Duda, R. O. and Hart, P. E. "Uses of the Hough Transformation to detect lines and curves in pictures", 15 Comm ACM, 1972, p. 11; and "Kimme et al., "Finding Circles by an Array of Accumulators", .sup.18 Comm ACM, 1975, p 120. Kimme describes the use of the generalized Hough technique to recognize circles in x-rays, something that is useful where tumors are spherical. These techniques differ from the present invention because they seek only details of pre-determined configuration.